This invention relates to acoustic systems and more particularly to acoustic systems having transducers, such as speakers in active noise control systems, operating in a wet environment or otherwise subject to condensation.
When used in an environment where the air is saturated with water vapor, transducers for active noise control systems must either be unaffected by the moisture or impervious to it. Conventional loudspeakers consist of a rigid cone suspended from a frame by a flexible surround. While it is possible to construct the cone from a rigid material, such as stainless steel, that is impermeable to water vapor, the surround must be flexible and is normally substantially permeable to water vapor. Loudspeakers have been constructed whose cones and surrounds were fabricated from titanium, and thus were impermeable to water vapor. However, the cone excursion was severely limited.
Alternative solutions use conventional loudspeakers with protective coatings, or elastic membranes in front of the speakers and the microphones. Similar to the problem with the flexible surround, it has proven difficult to design a coating or membrane with sufficiently low vapor transmission, low acoustic losses, low acoustic mass, high linearity of compliance, long excursion and long service life. The problem is compounded by the fact that the inner surface of the transducer enclosure is typically cooler than the duct interior. Water vapor diffuses through the speaker or protective membrane and condenses in the cooler transducer enclosure. Over time, water accumulates in the transducer enclosure.
An additional difficulty is that it is not practical to operate a totally sealed transducer enclosure in a system that operates at a static pressure different than the pressure in the enclosure. This is particularly true for loudspeakers where, due to the low compliance, static pressure differences of only a few inches of water will displace the cone and voice coil out of its operating region. This not only prevents the normal operation of the speaker, but, when carried to extremes, can damage it. The pressure difference between the inside of the transducer enclosure and the duct interior can increase during operation due to temperature changes, duct pressure changes and power dissipation inside the enclosure itself. It is therefore desirable to equalize pressure between the transducer enclosure and the duct interior.